Nearly 5,000 cave paintings found in northeastern Mexico were made by at least three groups of hunter-gatherers who lived in an area previously thought to have been uninhabited, archeologists said.

The drawings were discovered in 2006 in a number of caves in the San Carlos mountain range near the modern-day town of Burgos, about 100 miles south of the U.S. border. The first findings about the red markings that depict people, shapes and designs were revealed this week at a conference in Mexico City.

Mexico's National Institute of Anthropology and History has been cataloging and studying the drawings for the past two years.

Archeologist Martha Garcia Sanchez said the paintings include human and animal shapes, and relate to astronomy. They're abstract in nature, she said.

"Their importance is that based on them, we have been able to document the presence of pre-Hispanic groups in Burgos, where before it was believed there was nothing, when in reality it was inhabited by one or several cultures," she said.

Burgos is in Tamaulipas state, which borders Texas.

The ages of the cave paintings remain uncertain for now because archaeologists have not found any objects in the area that could help date them, said another archeologist, Gustavo Ramirez. Rains have also swept away sediment in the area, making that method of dating unavailable, too, he said.

At least three nomadic hunter-gatherer groups -- the Guajolotes, Iconoplos and Pintos -- are believed to have made the 4,926 drawings.

The images indicate that the groups prioritized hunting, fishing and gathering, Garcia said. Other drawings appear to depict religious and astronomical themes, she said.

Drawings of what appear to be teepees, local plants and animals like deer, lizards and centipedes can tell researchers about how the populations lived.

Drawings in one cave depicted an atlatl, a pre-Hispanic hunting tool that had not been previously found in other cave art in this part of Mexico, Garcia said.

Little is known about the indigenous groups that lived in the mountains near Burgos, she said.

These groups evaded Spanish colonists for nearly 200 years, she said, because they had the means to survive in the mountains, where the Spanish wouldn't venture.